When is it ever said that there are billions of multiverses in SMT. From what I've seen SMT contains like 5-8 multiverses, and the Amala Network is one multiverse (three at most), in which billions of universes are made and destroyed every moment. And even though time & space was destroyed, when was it ever said to have been destroyed on the conceptual level?

Beatrice, a character weaker than Battler is effortlessly able to create and destroy an entire infinite multiverse. In fact a manga scan (the manga being completely canon BTW) shows her holding the entire multiverse in her hands.